Drug-eluting implantable medical devices have become popular in recent times for their ability to perform their primary function (such as structural support) and their ability to medically treat the area in which they are implanted.
For example, drug-eluting stents have been used to prevent restenosis in coronary arteries. Drug-eluting stents may administer therapeutic agents such as anti-inflammatory compounds that block local invasion/activation of monocytes, thus preventing the secretion of growth factors that may trigger VSMC proliferation and migration. Other potentially anti-restenotic compounds include antiproliferative agents, such as chemotherapeutics, which include rapamycin and paclitaxel. Other classes of drugs such as anti-thrombotics, anti-oxidants, platelet aggregation inhibitors and cytostatic agents have also been suggested for anti-restenotic use.
Similarly, therapeutic agents have been added to orthopedic devices such a joint replacement prostheses, bone screws, staples and the like. Risks that may follow the placement of such devices include infection and, in the long term with some types of devices, loss of bone tissue at the interface with the device as the bone remodels and consequent loosening of the device. Therapeutic agents in such orthopedic devices may promote and/or inhibit bone and/or other tissue growth, inhibit rejection of the device or of components connected to or located adjacent to the device, reduce infection, reduce inflammation, reduce pain, provide vitamins and/or minerals, promote healing of surrounding tissue, and other similar functions.
Surgical staples have also included therapeutic agents to assist healing, prevent infections, or prevent other possible harmful effects of the surgical staple. Agents such as antimicrobial agents, anticoagulants, antithrombotic agents, antiplatelet agents, antiproliferative agents, anti-inflammatory agents, lipid-lowering agents, specific growth factor antagonists, antioxidants, genetic materials, angiogenic growth factors, antihypertension drugs, radioactive compounds, lymphokines, and other suitable agents have been proposed.
Leads connected to implantable cardioverter defibrillators are positioned inside the heart or on its surface. These leads are used to deliver electrical shocks, sense the cardiac rhythm and sometimes pace the heart, as needed. At the end of the leads are spiral wound metallic electrodes that are placed into the heart tissue, and deliver the electrical stimulation to the heart. It has been found that the natural immune response to a foreign body in tissue generate chemical species (typically peroxy compounds) that greatly increase corrosion of the metallic electrode. To thwart this immune response and the associated corrosion, electrodes are coated with anti-inflammatory drugs such as steroids, antibiotics, anti-fungal materials and the like by spray or dip coating methods. However, such coatings may be damaged during delivery to and insertion into the heart tissue. Further, it is sometimes difficult to deliver precise dosages of the drug.
The drug-eluting medical devices discussed above may be coated with a polymeric material which, in turn, is impregnated with a drug or a combination of drugs. Once the medical device is implanted at a target location, the drug(s) is released from the polymer for treatment of the local tissues. The drug(s) is released by a process of diffusion through the polymer layer for biostable polymers, and/or as the polymer material degrades for biodegradable polymers.
Controlling the rate of elution of a drug from the drug impregnated polymeric material is generally based on the properties of the polymer material. However, at the conclusion of the elution process, the remaining polymer material in some instances has been linked to an adverse reaction with the vessel, possibly causing a small but dangerous clot to form. Further, drug impregnated polymer coatings on exposed surfaces of medical devices may flake off or otherwise be damaged during delivery, thereby preventing the drug from reaching the target site. Still further, drug impregnated polymer coatings are limited in the quantity of the drug to be delivered by the amount of a drug that the polymer coating can carry and the size of the medical devices. Controlling the rate of elution using polymer coatings is also difficult.
Accordingly, drug-eluting medical devices that enable increased quantities of a drug to be delivered by the medical device, and allow for improved control of the elution rate of the drug, and improved methods of forming such medical devices are needed.